Methods for transforming the yeast Phaffia rhodozyma have been disclosed in European patent application 0 590 707 A1. These methods involve incubation of protoplasts with DNA or incubation of Phaffia cells with DNA followed by lithium acetate treatment. The recombinant DNA used to transform Phaffia strains with either of these methods comprised a Phaffia actin gene promoter to drive expression of the selectable marker genes coding for resistance against G418 or phleomycin. The methods involve long PEG and lithium acetate incubation times and transformation frequencies are low. When protoplasts are used, the transformation frequency is dependent on the quality of the protoplast suspension, making the procedure less reliable.
Recently a method for transforming Phaffia strains has been reported by Adrio J. L. and Veiga M. (July 1995, Biotechnology Techniques Vol. 9, No. 7, pp. 509-512). With this method the w transformation frequencies are in the range of 3 to 13 transformants per .mu.g DNA, which is low. A further disadvantage of the method disclosed by these authors consists in increased doubling time of the transformed cells. The authors hypothesised that this may be due to interference of the autonomously replicating vector with chromosome replication.
Clearly, there is still a need for a reliable and efficient method of transforming Phaffia strains with foreign DNA. It is an objective of the present invention to provide methods and means to achieve this. It is a further objective of the invention to optimize expression of certain genes in Phaffia rhodozyma in order to make Phaffia a more suitable production host for certain valuable compounds.